1. Field of the Invention: The invention relates to a method and apparatus for extracting core samples from any selected location in the side wall of a subterranean well.
2. History of the Prior Art: The taking of core samples during the drilling of a well is a time honored procedure. The prevailing practice has been to interrupt the drilling process, run an annular cutting tool into the bottom of the well, which tool is suspended beneath a core collecting barrel. Rotation of the annular cutting tool will effect the cutting of the core from the central portions of the well bottom which is trapped in the core barrel and removed to the surface for examination.
The difficulty with this method of obtaining core samples is that it is extremely time consuming in that you must interrupt the utilization of the rig for drilling purposes in order to take the core sample. As a result, the drilling rig operator has to be fairly certain as to where he wants core samples taken so as to minimize the number of times that the drilling operation is interrupted.
A better procedure is to drill in the entire well and then extract samples at any selected location through the utilization of side wall coring tools. These tools have heretofore taken the form of a hollow bullet which is forced radially outwardly by the discharge of an explosive propellant charge. The annular bullet embeds in the side wall of the well and has recovery cables connected to it which are then manipulated by an up and down movement of the work string to effect the loosening of the bullet from the side wall and the retrieval of the annular bullet, with the core sample contained therein, to the well surface. This method of obtaining core samples inherently results in the extraction of a very pulverized sample due to the fact that the bullet is impacted against the side wall with a large amount of kinetic energy. Obviously, the maximum energy imparted to the bullet occurs immediately after the discharge of the propellant charge. As a result both the rock grains and the sand grains are shattered, and the resulting core sample is not typical of the virgin condition of the rock. This drastically reduces the quality of the analysis that can be made from the crushed rock samples.
Another prior art approach to procuring said wall core samples was to incorporate diamond bladed saws or hollow rotary drills on the bottom of a wire line which were driven by small electric motors. Obviously, this requires the running of electric wire line which must continually carry considerable current, thus reducing the ability to be spliced and also reducing wire line life, itself. Furthermore, such prior art approaches resulted in the wire line carrying the entire tool weight, which further adversely effected wire line life and integrety. Moreover, such prior art devices also resulted in the material cutout of the sidewall being sometimes differentially stuck to the well bore and, therefore, difficult to recover. Furthermore, during the time in which it takes to drill these samples, the tool is static in the well and thus may contribute to its own differential sticking in the well, posing still another additional problem.
It follows that there is a definitive need in the well drilling art for a method and apparatus for efficiently extracting core samples from the side wall of a drilled well which will not result in the crushing of the sample, and which will readily retract the sample, once cut from the side wall, and permit it to be carried to the surface in an intact, almost virgin condition.